The present invention concerns a display utilizing the so-called electrochromism phenomenon, that is, one that manifests variations in the light absorption property upon application of properly controlled voltage. This display is referred to hereinafter as an "electrochromic display (ECD)".
A scheme of ECD is as follows. Through an electrolyte providing ion conductivity, an electrochromic material deposited on one transparent electrode, for example, a thin film of a transition metal oxide material, is electrochemically coupled with another electrode. When a thin film of the transition metal oxide material is supplied with a negative potential with respect to a specific potential, the current which flows through the oxide film is seemingly reduced, with the result that the oxide film is switched from a first absorption state to a second absorption state. On the contrary, when a positive potential is applied thereto the film of the transition metal oxide material is returned from the second absorption state to the first absorption state. While the film of the transition metal oxide material placed in the first absorption state does not exhibit absorption within a range of visible light (namely, transparent), the same in the second absorption state does exhibit a variety of absorption properties (namely, the film becomes colored). In the case where the film of the transition metal oxide material consists of an evaporated layer of amorphous tungsten oxide (WO.sub.3), the colored states thereof are explained below.
When cations of the electrolyte are a protons (H+),
______________________________________ xH.sup.+ + WO.sub.3 + xe.sup.- .revreaction. H.sub.x.sup.+ WO.sub.3 . e.sup.- x (transparent) (colored) ______________________________________
When the same are metal (M.sup.+) alkali metal is conventionally employed,
______________________________________ xM.sup.+ + WO.sub.3 + xe.sup.- .revreaction. M.sub.x.sup.+ WO.sub.3 . e.sup.- x (transparent) (colored) ______________________________________
A typical example of an ECD cell is illustrated in FIG. 1. A counter electrode is denoted as 1. This is made of a transparent indium oxide, electrically conducting film (obtainable through conventional vacuum evaporation method) or a NESA film (SnO.sub.2 ; obtainable through the conventional vacuum evaporation method or the spray method) for the transmission mode. This may be substituted by a layer of noble metal materials such as graphite, platinum and palladium. A substrate 2 may be formed of plastics, metal, ceramics, etc. The ECD cell further comprises a spacer and sealant resin 3. The transparent electrode 4 and the layer of the transition metal oxide 6 form in combination, a display electrode bearing a predetermined display pattern. Another transparent subtrate 5 may be made of glass, plastics or the like. A reference electrode 8 is made of the same material as the counter electrode 1. An electrolyte 7 is injected between the pair of the substrates 4 and 5.
The present invention is interested in the electrolyte in the ECD cells using tungsten oxide as the transition metal oxide material.
The prior art electrolytes which have been suggested include: (1) sulfuric acid solution of 1,2,3 propanetriol (for example, U.S. Pat. No. 3,708,220); (2) sulfuric acid solution of water and 1,2-ethanediol (for example, B. W. Faughnan et al., RCA Rev. 36 (4), 177 (1975)); and (3) propylene carbonate (for example, SID 75 Digest, 50, 1975). However, these prior art electrolytes exhibited the following disadvantages.
(1) lithium perchlorate
A glass substrate carrying an In.sub.2 O.sub.3 transparent electrode is heated at 150.degree. C. within a vacuum environment of 5 .times. 10.sup.-4. WO.sub.3 having a thickness of 5000A is deposited thereon with the evaporation rate of 10A/sec. 3.6 Normal sulfuric acid solution of 1,2,3 propanetriol-sulfuric acid is employed as the electrolyte to complete the fabrication of the transparent mode ECD cell. The many characteristics of the electrolyte are evaluated in the following manner.
The response characteristics were examined under the conditions that the display electrode of the ECD cell is grounded and the reference electrode (material: In.sub.2 O.sub.3) is kept at .+-.1.0V through the use of a constant potential driving technique. The results of the inventors' experiments reveal that the response is very poor and the ECD cell is not recommended as a proper timepiece display because the write response (say, a period of time required for reducing transmittancy from 100% to 30%) is 3.0 sec and the erase response (say, a period of time required for reducing transmittancy from 30% to 90%) is 4.0 sec at a temperature of 25.degree. C. Although the response characteristics may be improved correspondingly with an increase in applied voltage, hydrogen overvoltage of protons is conventionally only 1.5V in a water solution and probably even in a glycerine solution (the inventor's simplified experiments justify such an assumption). For this reason, if a voltage higher than the above named voltage is applied, then hydrogen gas will be produced about the counter electrode, thereby degrading the ECD cell.
(2) Sulfuric acid aqueous solution-ethylene glycol
The electrolyte employed is a mixture of sulfuric acid of 2 Normal concentration into a water solution of ethylene glycol (20%). The ECD cell is completed in the same manner as the above paragraph (1). Similarly, the response characteristics are evaluated when the reference electrode is driven at .+-.1.0V. The results are that the write response is 0.8 sec and the erase response is 0.4 sec. Therefore, the response characteristics are considerably superior to the above example (1). However, in the case where this system of the ECD cell is allowed to stand for a long time, for example for one day to several days, the WO.sub.3 film or the In.sub.2 O.sub.3 electrode tends to dissolve with accompanying degradation in the ECD cell.
(3) Lithium perchlorate, propylene carbonate
A 1.0-molar lithium perchlorate solution of propylen carbonate is used as the electrolyte. Since the redox potential of lithium ions (Li.sup.+) is about 3.0V, such a system of this electrolyte has the advantage that higher voltages are applicable in driving the ECD cell. When the reference electrode is a constant potential driven at .+-.1.5V, the write response is 0.6 sec and the erase response is 0.6 sec at 25.degree. C. Similarly, the WO.sub.3 film with a thickness of 5000A tends to dissolve in several days. In the event that sodium tungstate is previously saturated within the electrolyte, the operating life will be extended for an additional several days. Anyway, the operating life is at most 13 or 14 days.